This invention relates to rotary drill bits for drilling oil wells and the like, and more particularly to rotary drill bits used in conjunction with the drilling fluid circulation system of a rotary drill rig.
This invention involves an improvement over rotary drill bits of the type, such as that shown, for example, in U.S. Pat. Nos. 3,984,158 and 4,106,577, British Pat. No. 1,104,310 and FIG. 5 of this application, comprising a bit body having an upper portion adapted to be detachably secured to a drill string for rotating the bit, a chamber therein receiving drilling fluid under pressure from the drill string, and three depending legs each having an inwardly extending bearing journal. The bit further includes a roller cutter rotatably mounted on each bearing journal and three nozzles extending down between sets of adjacent roller cutters. The drill bit is used in conjunction with the drill fluid circulation system of a drill rig, with the drilling fluid being pumped down through the passage in the drill pipe to the chamber in the drill bit, exiting the drill bit via the nozzles, and flowing back up to the surface in the annulus around the drill pipe. The nozzles direct the drilling fluid as a high velocity stream against the bottom of the well bore to clean it, with the splash back of the drilling fluid from the bottom of the well bore impinging the roller cutters to provide limited cleaning action of the cutters. Drilling debris and cuttings from the bottom of the well bore are entrained in and are carried away from the bottom by the drilling fluid as it flows up the annulus.
While the above-described conventional drill bits have been satisfactory for drilling relatively brittle formations, they do not provide satisfactory rates of penetration when drilling relatively plastically deformable formations. Many commonly encountered formations such as slates, shales, limestones, sandstones and chalks, become plastically deformable under so-called differential pressure conditions, which occur when the hydrostatic pressure of the column of drill fluid bearing on the bottom of the well bore exceeds of the pressure of the formation surrounding the bore, as may happen in deep hole drilling. Whereas, brittle formations crack or fracture under the compressive loads applied by the cutting elements of a drill bit, plastic formations tend to deform and thus remain intact under such loads. In addition, certain of these plastic formations tend to form a relatively thick coating of drilling debris on the roller cutter which can result in so-called "bit-balling" and limited penetration of the formation by the cutting elements.
As shown in U.S. Pat. No. 4,106,577 and British Pat. No. 1,104,310, attempts have been made to increase the rate of penetration in plastic foundations by using extended nozzles for improving the cleaning action of the fluid circulation system. While this nozzle arrangement may offer some measure of improved cleaning action, it is still not satisfactory for many types of plastic formations. Moreover, in this arrangement, the nozzles extend down to points closely adjacent to the bottom of the well bore, and thus are subject to damage by irregularities, such as projections or ridges, on the bottom of the well bore, which may form from time to time during drilling operations.